fluid cavity

I am modelling an inflated sphere. The sphere is composed off two circular planar membranes joined together at the boundries. 

I have modelled the inside of the sphere as a fluid cavity and will inflate it to its threshold limit. 



I have achieved this by using the fluid cavity and fluid inflator method but for some reason when I run my input file I receive the following message: 



 ***ERROR: THE FLUID INFLATOR ACTIVATION HAS BEEN SPECIFIED ON THE FLUID